title: Synergistic activation by Ca2+ and Mg2+ as the primary cause for hysteresis in the phosphorylase kinase reactions.
abstract: A synergistic activation of phosphorylase kinase by Ca2+ plus Mg2+ was found to be the primary cause of the hysteresis, or lag, in the phosphorylase kinase reaction. Preincubation of the enzyme for short times with Ca2+ plus Mg2+ resulted in an approximately 7-fold increase in the kinase activity in subsequent assays with phosphorylase b or phosphorylase kinase as substrates, whereas preincubation with each metal ion by itself had no effect. Maximal activation through preincubation with Ca2+ plus Mg2+ occurred in 1 min 45 s and was readily reversed by chelation of both metal ions. As a result of the activation, the progress curve of phosphorylase b conversion at pH 6.8 was found to be nearly linear. Activation by Ca2+ plus Mg2+ was not apparent when subsequent assays were carried out at pH 8.2, or when previously autophosphorylated enzyme was used. Furthermore, the synergistic activation was found to occur significantly slower and/or to decrease in the presence of ATP, phosphorylase b, beta-glycerophosphate, and inorganic phosphate. How the synergistic activation by Ca2+ plus Mg2+ relates to autophosphorylation and the lag in the phosphorylase kinase reaction is discussed.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D004357:Drug Synergism; D004789:Enzyme Activation; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D009132:Muscles / Q000201:enzymology*; D010764:Phosphorylase Kinase / Q000378:metabolism*; D010763:Phosphorylase b / Q000378:metabolism; D010766:Phosphorylation; D011817:Rabbits
chemical_list: D010763:Phosphorylase b; D010764:Phosphorylase Kinase; D008274:Magnesium; D002118:Calcium